1. Technical Field
The present invention relates to a liquid crystal display device, and in particular to an active matrix liquid crystal display device which can prevent destruction due to an electrostatic discharge through a sealing material.
2. Prior Art
First, the structure of a prior active matrix liquid crystal display device will be explained referring to FIGS. 1 to 5. FIG. 1(A) is a top view of an active matrix liquid crystal display device having an array substrate 10 and a color filter (CF) substrate, i.e., an opposing substrate 12, and FIG. 1(B) is a side view. FIG. 2 is a diagram showing the structure of the array substrate 10. FIG. 3 is a cross-sectional view of the liquid crystal display device at the position indicated by an arrow delimited line 3--3 in FIG. 2; FIG. 4 is a cross-sectional view of the liquid crystal display device at the position indicated by an arrow delimited line 4--4 in FIG. 2; and FIG. 5 is a cross-sectional view of the liquid crystal display device at the position indicated by an arrow delimited line 5--5 in FIG. 2.
As is shown in FIG. 2, the array substrate 10 includes an array of display electrodes or pixel electrodes 16 that are formed on a glass support substrate 11, and thin-film transistors (TFTs) 14 provided correspondingly to the pixel electrodes 16. The pixel electrodes 16 are made of transparent metal, such as indium tin oxide (ITO). In this prior art, as is shown in FIG. 3, each of the TFTs 14 has an inverted staggered structure in which a gate electrode 18 is disposed below a semiconductor layer 19. The gate electrodes 18 are formed on the support substrate 11, and connected to gate lines 26, which are also formed on the support substrate 11. The gate lines 26 extend in horizontal direction outward beyond the location of a sealing material 40, and are connected to terminal connecting pads 38 for outer lead bonding (OLB). A silicon oxide insulating layer 28, which also serves as a gate insulating layer, is formed on the gate electrodes 18 and the gate lines 26. The semiconductor layer 19, made of amorphous silicon or polycrystalline silicon, is formed at a position corresponding to that of the gate electrode 18. A drain electrode 20 and a source electrode 22 are formed in contact with the semiconductor layer 19. The source electrode 22 is connected to the associated pixel electrode 16. The entire surface, excluding the pixel electrodes 16, is covered by an insulating layer 30 such as silicon nitride.
As is shown in FIG. 2, the drain electrodes 20 are connected to data lines 24, which extend in vertical direction outward beyond the location of the sealing material 40, and are connected to terminal connecting pads 36 for OLB. The pads 36 and 38 are connected to a TAB circuit substrate in order to drive and control the liquid crystal.
The opposing substrate 12 includes a common electrode 34 made of transparent metal, such as ITO, that is formed on a support substrate 13 such as a glass substrate, and a color filter (not shown). The opposing substrate 12 is disposed at a predetermined distance opposite the array substrate 10. The polymer sealing material 40, such as epoxy resin, is so located that it surrounds a display area which includes the pixel electrodes 16 and the TFTs 14. The sealing material 40 normally contains glass filament spacers for establishing the distance between the array substrate 10 and the opposing substrate 12. A liquid crystal material 32 is introduced into the area sealed by the sealing material 40.
As is shown in FIGS. 4 and 5, the insulating layer 28 extends outward beyond the location of the sealing material 40 to the vicinity of the pads 36 and 38, and serves as a lower protective insulating layer that protects the gate lines 26 and separates the gate lines 26 and the data lines 24. The insulating layer 30 serves as an upper protective insulating layer that protects the surface of the array substrate 10. The upper insulating layer 30 also extends beyond the location of the sealing material 40 to the vicinity of the pads 36 and 38. The sealing material 40 positionally overlaps the common electrode 34.
It has been found that a liquid crystal display device tends to accumulate an electrostatic charge during the fabrication process, or during a period from the fabrication to installation in a final product, such as a personal computer, and causes an electrostatic discharge through the sealing material 40. Such an electrostatic discharge occurs only between the common electrode 34 and the data line 24, and does not occur between the common electrode 34 and the gate line 26. As is apparent from FIG. 4, the gate lines 26 are positioned between the lower insulating layer 28 and the support substrate 11, and the two insulating layers 28 and 30 exist between the sealing material 40 and the gate lines 26. On the other hand, the data lines 24 are positioned between the insulating layers 28 and 30, and only the insulating layer 30, which is about 1000 .ANG. thick, is present between the sealing material 40 and the data lines 24. Therefore, an electrostatic discharge occurs preferentially between the common electrode 34 and the data line 24. It has been found that the electrostatic discharge occurs even when an insulating layer 30 as thick as 2000 .ANG. is employed.
The electrostatic discharge occurs between the common electrode 34 and the data line 24 through the sealing material 40 and the upper insulating layer 30. When an electrostatic discharge occurs, the thin data lines 24 may be destroyed, or a short-circuit may occur between the common electrode 34 and the data line 24 due to the carbonization of the resin in the discharge path and/or due to the dispersion of data line metal. This results in a reduction in the yield.
As a result of analysis, it has been found that the dielectric constant of a particulate filler mixed in the resin sealing material, the shape and the size of the filler, and the distribution of the filler in the sealing material are related to the electrostatic discharge. To form the sealing material, for example, a seal application tool extrudes a resin material and applies it in a frame shape to one substrate, then places the other substrate thereon and compresses the seal material. Generally, a filler consisting of an inorganic dielectric material is mixed with the resin in order to ensure the stability of the shape of the sealing material after it is applied to or compressed on the substrate. When the width of the sealing material is not uniform after it has been compressed, a margin space large enough to accommodate the change in the width of the sealing material must be provided between the sealing material and the outermost pixels, and the effective display area can not be increased. The employment of a filler, therefore, is required to provide better control of the shape of the sealing material.
Typically, silica particles are used as the filler. Silica is generally spherical because of its inherent crystal structure. A spherical filler, however, is not so good in terms of the control of the shape of the sealing material. To improve the shape controllability, a plate-like filler such as talc [Mg.sub.3 Si.sub.4 O.sub.10 (OH).sub.2 ] or a filler of variable or irregular shape, such as alumina, is preferable. Of these, the plate-like filler is more desirable. However, these fillers have a dielectric constant that is higher than that of the resin material, and may cause a problem of electrostatic discharges, which seldom occur with a silica filler.
Although the mechanism of the electrostatic discharge through the sealing material is not yet fully understood, the mechanism is considered to be based on the following. Silica has a dielectric constant (approximately 3.5 to 4.5) that is similar to the dielectric constant (approximately 3.5 to 5.0) of epoxy resin, which is a sealing material. Therefore, even when silica particles are distributed unevenly in the epoxy resin, the dielectric constant of the sealing material is substantially uniform, and accordingly the potential distribution is also uniform. On the other hand, since the dielectric constant of talc is approximately 9 and the dielectric constant of alumina is approximately 8 to 10, these are considerably higher than the dielectric constant of epoxy resin. Furthermore, when a plate-like filler or a filler of variable shape is employed, the size of the filler must be increased to a degree in order to preferably control the shape of the sealing material. A large filler tends to be non-uniformly distributed in the sealing material. Thus, when a filler such as talc or alumina is distributed non-uniformly in epoxy resin, the dielectric constant of the sealing material is non-uniform and the potential distribution is also non-uniform.
FIG. 6 is a schematic diagram illustrating a state of plate-like talc filler particulates 44 in the sealing material 40. The filler particulates 44 are non-uniformly distributed in epoxy resin 42, and three filler particulates overlap at a position 46 where the thickness of the epoxy resin is very thin. Consider a simplified model where a resin layer having dielectric constant Er and thickness dr, and a filler layer having dielectric constant Ef and thickness df are laminated. Assuming that the capacitance of the resin layer is Cr, a voltage applied across the resin layer is Vr, the capacitance of the filler layer is Cf, a voltage applied across the filer layer is Vf, the area is S, and V=Vr+Vf, then, since Q=Cr.multidot.Vr=Cf.multidot.Vf, Cr=Eo.multidot.Er.multidot.S/dr and Cf=Eo.multidot.Ef.multidot.S/df (Eo is a dielectric constant in vacuum), the following relationship can be obtained:
Vr:Vf=Cf:Cr=(Ef/df):(Er/dr)
The voltage applied across the resin is proportional to the ratio R(Vr)=Vr/V=Vr/(Vr+Vf). EQU R(Vr)=(Ef/df)/[(Ef/df)+(Er/dr)]=(Ef.multidot.dr)/(Ef.multidot.dr+Er.multido t.df)
The voltage applied across the resin of unit thickness is proportional to ratio R(Vr)/dr. EQU R(Vr)/dr=Ef/(Ef.multidot.dr+Er.multidot.df)=1/[dr+(Er.multidot.df/Ef)]
Therefore, the voltage applied across the resin of unit thickness is increased as the dielectric constant Ef of the filler becomes greater, and the thickness dr of the resin becomes smaller. Of course, the simplified model does not correspond to the actual sealing material, but can be regarded as representing a tendency in the characteristic. The thin epoxy resin portion at the position 46 will have a greater potential difference per unit thickness than that of other epoxy resin portions. In addition, the lowest filler particle has a small protrusion at the position 46. It is considered that the very thin resin layer at the position 46 itself, or in combination with other factors, such as the small protrusion on the filler, forms a "peculiar point" in the sealing material, which can induce an electrostatic discharge. The discharge will pass through the resin at the position 46 and travel along the surface of the filler. If such a "peculiar point" is formed by the alumina filler of variable shape, an electrostatic discharge can occur.
If the size of the high dielectric filler is reduced and the filler is uniformly distributed throughout the resin sealing material, the above mentioned electrostatic discharge will be prevented. In this case, however, there is deterioration of the capability to control the shape of the sealing material. Also, if the silicon nitride layer 30 in FIG. 5 is formed sufficiently thicker than 2000 .ANG., the electrostatic discharge will be prevented, even with the structure in FIG. 5. However, the employment of a thick insulating layer 30 is not preferable because longer time will be required for the deposition and patterning processing. Therefore, there is a demand for a solution that makes it possible to easily avoid the electrostatic discharge problem, even when a high dielectric constant filler, in particular, a plate-like filler, having a superior shape controllability is employed.
Japanese Unexamined Patent Publication No. (Patent Kokai No.) 08-29794 (1996) discloses a liquid crystal display device which uses a sealing material consisting of an electrically conductive resin in order to prevent destruction due to an electrostatic discharge.
Japanese Unexamined Patent Publication No. (Patent Kokai No.) 09-152620 (1997) discloses a liquid crystal display device which uses a combination of a sealing material containing electrically conductive spacers and a shunt transistor to prevent destruction due to an electrostatic discharge.
Japanese Unexamined Patent Publication No. (Patent Kokai No.) 05-134261 (1993) discloses a liquid crystal display device that prevents a short-circuit between a common electrode and exposed data lines, which occurs through conductive foreign materials that are mixed in a sealing material or attached to the surface of the sealing material. The wiring route of the data lines located between the sealing material and an insulating film is changed so that they pass under the insulating film at the position of the sealing material, and thus do not contact the sealing material. This prior art, however, does not take into consideration an electrostatic discharge that occurs inherent to a high dielectric constant filler in the sealing material. The short-circuit due to the conductive foreign materials can be eliminated by removing the foreign materials, or by covering the exposed data lines with a thin insulating layer. The problem with the electrostatic discharge that the present invention intends to overcome, however, can not be solved by removing conductive foreign materials from the sealing material, or by covering the data lines with a thin insulating layer.
It is, therefore, one object of the present invention to provide a liquid crystal display device that can solve the electrostatic discharge problem resulting from a high dielectric constant filler that is appropriate for the improved shape stability of a sealing material.